1. Field of the Invention
This invention relates to improvements in and concerning a method for the production of a dielectric-separation substrate for use in a semiconductor integrated circuit having single crystal silicon for the substrate.
2. Description of the Prior Art
Generally, semiconductor integrated circuits are each composed of transistors, diodes, resistors, etc. formed in a limited area of a substrate and, therefore, are required to have some transistors, diodes, resistors, etc. devices electrically insulated and separated from the others. As methods for the separation of such devices, the PN junction separation and the dielectric separation are available, for example. The dielectric separation is advantageous over the PN junction separation because of a high dielectric strength and a small parasitic capacitance. The dielectric separation has been attracting attention in its practical use because this method allows production of a semiconductor integrated circuit enjoying high voltage breakdown, a large capacity, and a high operating speed.
The dielectric separation falls into two forms, one giving rise to a polycrystalline silicon deposition type dielectric-separation substrate and the other a bonded type dielectric-separation substrate. The polycrystalline silicon deposition type dielectric-separation substrate has the disadvantage that the deposition of polycrystalline silicon consumes much time and incurs a heavy cost because this deposition must be continued until the deposited polycrystalline silicon reaches a film thickness of about 500 .mu.m and, during the deposition, the substrate is warped in consequence of shrinkage of the deposited polycrystalline. In contrast, the bonded type dielectric-separation substrate is free from the disadvantage just mentioned and is highly useful in that it is manufacturable wit a substrate of a large diameter.
The conventional bonded type dielectric-separation substrate has been produced by a method which comprises bonding to a single crystal silicon substrate having an oxide film formed in advance thereon another single crystal silicon substrate through the medium of the oxide film, then grinding and polishing the single crystal silicon substrate and forming a separating groove therein by mesa etching, forming a separating oxide film, depositing thereon a polycrystalline silicon layer of a thickness greater than the depth of the groove thereby completely filling the groove, subsequently polishing the excess polycrystalline silicon layer until the oxide film in the area other than the separating groove is exposed, and thereafter removing the exposed oxide film by etching with hydrofluoric acid.
In the method described above, when the exposed oxide film is removed with hydrofluoric acid, however, the polycrystalline silicon layer 10 assumes the construction of hornlike projections and gives rise to stepped levels as illustrated in FIG. 2.
When the dielectric-separation substrate obtained as described above is used in manufacturing a semiconductor device, the projections are chipped off to give rise to particles in the process of manufacture and the projections have the possibility of inducing discontinuity or breakage of distributed wires after formation of the device.
This invention is aimed at solving the problem described above. An object of this invention is to provide a method for the production of a dielectric-separation substrate having no projection from the polycrystalline silicon layer.